Robots are widely used in various application fields like industrial robots, surgical robots, military robots, or the like. Actuators are essential elements for implementing motions of such robots, and, from among various actuators, motors are most frequently utilized for robots.
Motors are mainly used to rotate wheels of mobile robots or to move joints of manipulators. In this case, if a robot is implemented to be driven simply by a motor with a gearbox with a fixed gear ratio and the like without a special mechanism, it would be difficult to obtain wide speed-torque operating range and performance of the robot as desired by a developer and a user since a final nominal torque and a speed of the motor are fixed. In addition, the gearbox has a large volume and weight, which leads to an increase in the volume and weight of the entire drive system.
Therefore, if such a problem is not solved due to the performance of the actuator, a special mechanism for satisfying required performance according to each application field and a use purpose of a robot would be required.
As such a special mechanism, there is a twisted string driving mechanism which can generate a large linear driving force with a small torque. Twisted string actuation mechanism is rotary-to-linear transmission that produce a remarkably high transmission ratio by twisting a single string or a pair of strings. Therefore, it can substantially amplify the linear driving force using a very light string without the need for a bulky gearbox. However, this mechanism has a disadvantage that it also suffers from a narrow speed-force operating range like a motor with a gearbox.
Most of the robots currently developed in various fields require a great driving force and a rapid driving speed. However, due to the limit on the performance of the actuator, robots are developed by designers in such a manner that the driving force and the driving speed are traded off according to a purpose of use or use of robots, and as a result, robots having satisfactory performance are not developed.
Furthermore, there is a need for a method for changing a speed at a desired time while consistently satisfying a speed-changing condition of a speed-change mechanism, and also for a method for changing a speed rapidly.